Right now, the whole idea of Arcane mage revolves around having burn / conserve phases in between Evocation cooldowns. In this 1:30 minute window of in-between Evocation phase, I roughly calculated (from my own experience), we spend around 20-25 seconds in burn mode, and around 1:05-1:10 minute in conserve mode. The higher your Mastery levels are, the more seconds you spend in burn mode. The inverse is true for Haste.

By taking this line of thinking to the limit, I was flirting with the idea of stacking insane amounts of Mastery and zero haste, and picking 3x the new Azerite trait Equipose (which will lower the mana cost of Arcane Blast by 750 for each talent, which will be 2250 mana savings from each AB) - and maybe picking the new trait Arcane Pummeling (which will increase the chances of a Clearcast), then taking Incanter's Flow / Time Anomaly to have a longer burn phase. If done rightly, and with high enough gear levels, maybe even one huge perpetual burn phase, in which we never cast a single Arcane Barrage, and stay at 4 Arcane Charges the whole fight.

I am wondering if this is theoretically possible? How much mastery would be required to have around 180-200k mana pool and insane passive mana regeneration? Did anyone try this? Just how much your mana regen / mana pool will be, and how long your Burn phase can last, if you have like 3000 mastery, for instance?

At a certain point time becomes the limiting factor to the burn phase and not mana. You still need AP and ROP for the burn phase. Else you are just dumping your mana really. Same thing happened at the end of Legion where Arcane mages needed more haste because mastery reached values where time became the limiting factor instead of mana. I wouldn't bother stacking mastery.

As far as i know from ptr testing the manacost reduction does not stack. So having more than 1 of that trait is pretty much useless.
If you have more than 1 it is just a worse version of Galvanizing Spark.